1. Field of the Invention
The present invention relates generally to a hydraulic shock absorber for absorbing shock applied thereto and is applicable to a vehicle suspension, for example. More specifically, the invention relates to a hydraulic shock absorber which has fluid passages for establishing fluid communication between upper and lower fluid chambers defined by a piston of the shock absorber.
2. Description of the Prior Art
Generally, hydraulic shock absorbers have a cylinder filled with a working fluid, and a piston which divides the fluid chamber within the cylinder into upper and lower chambers. The piston is formed with a fluid passage which establishes fluid communication between the upper and lower chambers. A disk valve assembly comprising a plurality of elastic disk plates covers one end of the fluid passage. The inner portion of the disk valve assembly is fixed to the piston, and the outer portion thereof is in contact with the piston, so that a space is established between the disk valve assembly and the piston when the disk valve assembly is deformed due to the fluid pressure in the chamber of the cylinder. In such a disk assembly, fluid resistance is applied to the working fluid passing through the fluid passage, so that a predetermined damping force is applied to the piston.
In order to effectively apply a damping force to the piston when the amplitude of the piston is relatively large as well as very small, the Japanese Utility-Model First (unexamined) Publication (Jikkai Sho.) No. 60-52438 discloses an improved hydraulic shock absorber. The disclosed shock absorber has a disk valve assembly comprising a plurality of elastic disk plates, and an orifice establishing fluid communication between upper and lower chambers of the cylinder is formed in one disk plate which is brought into contact with the valve seat surface of the piston. By this orifice, flow resistance is applied to the working fluid passing through the fluid passage, so that a predetermined damping force can be applied to the piston even if the amplitude of the piston is very small.
However, in such a hydraulic shock absorber, difference between fluid pressures before and behind the disk valve assembly is relatively great. Therefore, when the working fluid which passes through the orifice has high flow velocity, vortex flow and cavitation are produced in the downstream chamber, thereby causing fluid noise of the working fluid.